(1) Field of the Invention
The present invention relates to an optical wavelength selective control apparatus for selecting arbitrary optical signals from wavelength-division-multiplexed optical signals and performing optical transmission.
(2) Description of the Related Art
As well known, wavelength-division-multiplexing (WDM) transmission system using band characteristics of an optical fiber is expected as a transmission system which can increase a transmission capacity or can configure an optical network whose flexibility has been improved because it is easy to drop/add a signal.
In concrete, the WDM transmission system wavelength-division-multiplexes plural optical signals at different wavelengths and transmits them over one optical fiber. Therefore, if multiplexing signals at the same transmission rate, the WDM transmission system can transmit a larger quantity of information by the number of times of wavelength-division-multiplexing than a transmission system which modulates, at a high speed, optical signals at one kind of wavelength and transmits them over one optical fiber. Even with respect to low-speed optical signals, the WDM transmission system can obtain the same transmission capacity as a transmission system which transmits high-speed optical signals at one wavelength, by wavelength-division-multiplexing the low-speed optical signals.
In the above WDM transmission system, wavelengths of transmitted optical signals are required to be spaced to one another to a degree that a signal is not affected by a signal at a neighboring wavelength. There is an optical amplifier having a band larger than ten-odd nanometers, at present. It is therefore possible to realize a WDM transmission system in which the above wavelengths are spaced approximately one nanometer, and such system is being introduced as a real system.
A lot of researches are conducted on optical networks based on the above WDM transmission system in these years. As an example, there is a network having an ADM (Add-Drop Multiplex) function of not only transmitting WDM signals from one point to another point, but also selectively transmitting only an optical signal at a specific wavelength among multiplexed optical signals at a repeating point called a node provided in the course of a transmission line, or receiving signals at other wavelengths at the node, or adding a light of another signal at the node and transmitting the signal to another node. The ADM function is a technique having a feature that can drop/add a signal as in a state of light at will, which is characteristic of the WDM transmission technique.
As an important device for the above WDM transmission system, there is a wavelength division selective optical filter (hereinafter called merely an optical filter, occasionally). For example, there is used the optical filter on the receiving side of the WDM transmission system in order to split wavelength multiplexed optical signals (WDM signals) by wavelength and receive it. The wavelength division selective optical filter in this case eliminates unnecessary signals (other signals) other than a transmitted optical signal, and eliminates noises generated by an optical amplifier provided along the transmission line, at the same time. For this reason, the optical filter is generally required that a transmission band for each wavelength is narrow as much as possible in order to suppress other signals or noises, or a wavelength to be selected is variable in order to select a signal at an arbitrary wavelength.
Such wavelength-selective optical filter is also used as, for example, an optical ADM node or an optical crossconnect apparatus (not shown) in an optical network. Since the optical ADM node needs a function of transmitting (adding) an optical signal at an arbitrary wavelength and receiving (dropping) an optical signal at an arbitrary wavelength, the optical filter is used on both the adding and dropping sides, where a wavelength to be selected is required to be variable.
In the optical crossconnect apparatus, the optical filter is used in a portion where an optical signal is converted from one wavelength to another wavelength. In order to transmit an electric signal with a light at an arbitrary wavelength, the optical filter selects only a light at a desired wavelength among transmittable N-wavelength-multiplexed CW (Continuous Wave) lights, and applies a transmit signal thereto.
As the wavelength selecting optical filter, there is an optical filter (AOTF: Acousto-Optic Tunable Filter) using the acoustooptic effect, for example.
FIG. 14 is a block diagram showing a structure of the AOTF. An AOTF 50 shown in FIG. 14 has a light input port 50a, an optical waveguide 501, polarization beam splitters (PBSs) 502 and 507, SAW absorbers 503 and 506, a finger electrode (IDT) 504, an SAW cladding (Ti-deeply-diffused region) 505 and light output ports 50b and 50c. In the AOTF 50, an optical signal propagated through the optical waveguide 501 interferes with a surface acoustic wave propagated through the SAW cladding 505 so that only a light at a part of wavelengths undergoes polarization-conversion. The splitter (PBS 507) splits only the polarization-converted light to take out (select) a part of the wavelengths.
In concrete, an RF signal corresponding to a light at a wavelength to be taken out is applied to the IDT 504 (electrode logarithm N, opening length W) to generate a surface acoustic wave (SAW) which applies polarization-conversion to only a light at a wavelength to be taken out, and the surface acoustic wave is propagated through the SAW cladding 505. At this time, a microwave is generated from the IDT 504 toward the both sides of the SAW cladding 505, which might affect a polarization-splitting process in the PBSs 502 and 507. However, the microwave is absorbed by the SAW absorbers 503 and 506.
When an optical signal is inputted from the input port 50a in this state, the optical signal is polarization-split by the PBS 502, and propagated as an optical signal in a TE mode and an optical signal in a TM mode through the optical waveguides 501a and 501b. 
Each of these signals interferes with the above surface acoustic wave propagated through the SAW cladding 505, whereby only an optical signal at a wavelength which is desired to be taken out is polarization-converted (TE-TM mode conversion). The optical signal so polarization-converted is polarization-split at the PBS 507 so that only an optical signal (selected optical signal) at a wavelength desired to be taken out is outputted from the light output port 50c. Optical signals having not been selected are outputted from the other light output port 50b. 
Here, if a temperature of the AOTF 50 (device) is constant in the wavelength selecting process at the AOTF 50, a relation between a frequency of the above surface acoustic wave and a frequency of the selected optical signal is 1:1. Accordingly, in the AOTF 50, when a frequency of the RF signal supplied to the IDT 504 is varied, the selected optical wavelength is varied. As this, a wavelength variable selective optical filter is realized with AOTF 50.
When a plurality of RF signals at different frequencies are mixed and supplied to the IDT 504, the AOTF 50 can select a plurality of light wavelengths corresponding to frequencies of the RF signals at a time. Namely, the AOTF 50 is very effective as an ADM (multiple wavelength selective optical) filter which can select not only one wave but also a plurality of optical signals at desired wavelengths simultaneously.
Hereinafter, description will be made of an actual system to which the AOTF 50 is applied.
FIG. 12 is a block diagram showing an example of a structure of a WDM transmission system. In a WDM transmission system 600A shown in FIG. 12, unnecessary components such as sidebands and the like of optical signals at different wavelengths generated by light sources (LDs) 610-1 through 610-n are eliminated by band-pass filters (BPF) 613xe2x80x2-1 through 613xe2x80x2-n (n is a natural number) in a sending system 61xe2x80x2. The optical signals are modulated by respective modulators (MOD) 61c-1 through 61c-n, multiplexed by a multiplexing coupler 61d, and transmitted to a receiving system 62xe2x80x2. Incidentally, the above BPFs 613xe2x80x2-1 through 613xe2x80x2-n also serve to eliminate an effect on other channels when wavelength variation occurs in the light sources 610-1 through 610-n.
In a receiving system 62xe2x80x2, a demultiplexing coupler 62d demultiplexes the optical signals from the sending system 61xe2x80x2, AOTFs 62a-1 through 62a-n having the same structures as the above AOTF 50 select respective optical signals at one wave, and each of receiving units 62b-1 through 62b-n receives an optical signal at a desired wavelength.
In the WDM transmission system 600A, the sending system 61xe2x80x2 and the receiving system 62xe2x80x2 have a transmit light source redundant unit 64 and a receiving unit redundant unit 65, respectively, functioning as a standby at the time of fault of a transmitting function or a receiving function corresponding to a certain wavelength, as shown in FIG. 12.
In concrete, the transmit light source redundant unit 64 has a tunable light source (T-LD) 610A of an output optical wavelength tunable type, an AOTF 613A similar to the above AOTF 50 and a modulating unit (MOD) 61cxe2x80x2. When a fault develops in any one of sending functions for respective wavelengths accomplished by respective combinations of the LDs 610-1 through 610-n, BPFs 613xe2x80x2-1 through 613xe2x80x2-n and MODs 61c-1 through 61c-n, an output wavelength of the tunable light source 610A and a selected wavelength of the AOTF 613A are switched to a wavelength at which the fault occurs so as to function in lieu of the portion in which the fault occurs.
The receiving unit redundant unit 65 has an AOTF 62A similar to the above AOTF 50 and a receiving unit 62B similar to the above receiving units 62b-1 through 62b-n. When a fault occurs in any one of receiving functions for respective wavelengths accomplished by respective combinations of the AOTFs 62a-1 through 62a-n and the receiving units 62b-1 through 62b-n, a selected wavelength of the AOTF 62A is switched to a wavelength at which the fault occurs so as to function in lieu of a portion in which the fault occurs, similarly to the above transmit light source redundant unit 64.
In the above WDM transmit system 600A, the sending system 61xe2x80x2 and the receiving system 62xe2x80x2 are provided with respective standbys using selected-wavelength alterable AOTFs, whereby reliability of the system is improved.
FIG. 13 is a block diagram showing an example of a structure of an optical ADM node. An optical ADM node 600 shown in FIG. 13 has an optical ADM unit 60, a sending system 61, a receiving system 62 and an optical amplifier 63.
The optical ADM unit 60 performs a dropping/adding process on transmit optical signals, which has an AOTF 60a similar to the above AOTF 50 as a wavelength selective optical filter for dropping an optical signal. Incidentally, the AOTF 60a can select optical signals at a plurality of wavelengths (select multiple wavelengths) according to frequencies of RF signals (f1, f2, . . . fx; xxe2x89xa6n) for selecting respective wavelengths.
The sending system 61 transmits optical signals at arbitrary wavelengths among transmittable N waves (N is a natural number) to the optical ADM unit 60 in order to add the optical signals to inputted optical signals to the optical ADM unit 60, which has, for example, a wavelength division multiplexed signal distributing light source 61a, gate switches 61b-1 through 61b-n (n is a natural number), modulators (MOD) 61c-1 through 61c-n, a multiplexing coupler 61d and an optical amplifier 61e. 
The wavelength division multiplexed signal distributing light source 61a generates and outputs optical signals at wavelengths to be added to inputted optical signals at the optical ADM unit 60. For this purpose, the wavelength division multiplexed signal distributing light source 61a has light sources (LDs) 610-1 through 610-n for outputting lights (signals) at different wavelengths xcex1 through xcexn, a multiplexing coupler 611 for multiplexing lights at respective wavelengths xcex1 through xcexn outputted from the LDs 610-1 through 610-n, a demultiplexing coupler 612 for demultiplexing the optical lights multiplexed by the multiplexing coupler 611 into n wavelengths xcex1 through 80n, AOTFs 613-1 through 613-n each for allowing a light at only a corresponding wavelength (xcex1, xcex2, xcex3, . . . or xcexn) among optical outputs demultiplexed by the demultiplexing coupler 612 to pass therethrough, and a wavelength stabilizing circuit 614. Incidentally, each of the above AOTFs 613-1 through 613-n is similar to the above AOTF 50, which is used as an optical filter for selecting one wave.
The sending system 61 can select and transmit an arbitrary number of optical signals at aribitrary wavelengths among transmittable N waves, by using the AOTFs 613-1 through 613-n.
The wavelength stabilizing circuit 614 monitors optical outputs demultiplexed by the above demultiplexing coupler 612, adjusts minute deviations or the like of the optical wavelengths outputted from the LDs 610-1 through 610-n, thereby stabilizing the wavelengths xcex1 through xcexn of the transmit optical signals.
As shown in FIG. 13, each of the gate switches 61b-1 through 61b-n is switched according to whether an optical signal at a wavelength xcex1, xcex2, xcex3, . . . or xcexn is newly added at the optical ADM unit 60 to a vacant wavelength resulted from that the optical signal at the corresponding wavelength xcex1, xcex2, xcex3, . . . or xcexn dropped at the AOTF 613-1, 613-2, . . . or 613-n, which are connected in series (cascade) to one another, is received by the receiving unit 62b-1, 62b-2, . . . or 62b-x (xxe2x89xa6n) to be described later. The modulators 61c-1 through 61c-n modulate optical signals having passed through the gate switches 61b-1 through 61b-n. The multiplexing coupler 61d multiplexes the optical signals modulated by the modulators 61c-1 through 61c-n. The optical amplifier 61e amplifies the optical signals multiplexed by the multiplexing coupler 61d. 
The receiving system 62 shown in FIG. 13 receives optical signals selected (dropped) by the optical ADM unit 60, which has AOTFs 62a-1 through 62a-x (x less than n) and receiving units 62b-1 through 62b-x. Each of AOTFs 62a-1 through 62a-x used here is similar to the above AOTF 50. In this case, each of the AOTFs 62a-1 through 62a-x is served as an optical filter for selecting an optical signal at one wavelength by being applied thereto an RF signal at one frequency.
Each of the receiving units 62b-1 through 62b-x receives an optical signal at a predetermined wavelength (xcex1, xcex2, xcex3, . . . or xcexx) dropped by the corresponding AOTF 62a-1, 62a-2, . . . or 62a-x.
In the case where WDM optical signals at wavelengths xcex1 through xcex8 are inputted to the optical ADM unit 60 and WDM optical signals at wavelengths xcex1 through xcex4 are selected in the AOTF 60a, the four AOTFs 62a-1 through 62a-4 select the wavelength xcex1 through xcex4 of the WDM optical signals and separate by wavelength to receive them in the receiving system 62.
Namely, the AOTF 62a-1 selects only an optical signal at a wavelength xcex1 corresponding to an RF signal at a frequency f1 among the WDM optical signals at wavelengths xcex1 through xcex4, and the receiving unit 62b-1 receives the selected optical signal. The AOTF 62a-2 selects an optical signal at a wavelength xcex2 corresponding to an RF signal at a frequency f2 among the WDM optical signals at wavelengths xcex2 through xcex4 having not been selected in the AOTF 62a-1, and the receiving unit 62b-2 receives the selected optical signal. The other AOTFs 62a-3 and 62a-4 select optical signals at wavelengths xcex3 and xcex4 corresponding to RF signals at frequencies f3 and f4, respectively, and the receiving units 62b-3 and 62b-4 receive the respective selected optical signals.
The optical amplifier 63 shown in FIG. 13 amplifies optical signals having not been selected by the optical ADM unit 60 (that is, optical signals to be transmitted to the next system) such that the optical signals can be transmitted for a predetermined transmission distance.
As stated above, the optical ADM node 600 can drop optical signals at arbitrary wavelengths from transmitted optical signals (WDM signals) at a plurality of wavelengths by the optical ADM unit 60 to receive them, or add optical signals at arbitrary vacant wavelengths to transmit them.
When the above receiving system 62 or 62xe2x80x2 (refer to FIG. 13 or 12) switches such that the AOTF 62a-1 drops (selects) first an optical signal at a wavelength xcex1, after that, drops an optical signal at a wavelength xcex5, a frequency of an RF signal to be supplied to the AOTF 62a-1 is switched from a value (f1) corresponding to the wavelength xcex1 to a value (f5) corresponding to the wavelength xcex5.
However, a frequency of the RF signal changes continuously from f1 to f5, so that a wavelength selected by the AOTF 62a-1 changes continuously with a change of the frequency. For this, the receiving unit 62b-1 receives optical signals at wavelengths xcex2, xcex3, xcex4, etc., other than the wavelength xcex5 until receiving the optical signal at a wavelength xcex5 that should be received. Therefore, it is quite inappropriate to switch a wavelength of a received signal while the system is operated.
Since the above optical ADM node 600 or an optical crossconnect apparatus, in particular, multiplexes optical signals at a plurality of wavelengths and transmits them after performing the above wavelength selecting process, if an optical signal at another wavelength xcex2, xcex3 or xcex4 is transmitted even in a moment when a selected wavelength is switched from xcex1 to xcex5 as above, an optical signal at the same wavelength as another optical signal at a wavelength other than a wavelength to be selected might leak. This affects an optical signal at another wavelength as coherent crosstalk, leading to degradation of the transmission characteristic.
For the purpose of solving the above problem, there has been proposed a technique disclosed in Japanese Patent Laid-Open Publication Number 7-199252.
FIG. 15 is a block diagram showing an example of an optical wavelength selective control apparatus for the purpose of explaining the above technique. An optical wavelength selective control apparatus 100 shown in FIG. 15 has an AOTF 50, an RF oscillator 51 and a control unit 52.
The AOTF 50 is similar to that described before with reference to FIG. 10. The RF oscillator 51 outputs an RF signal at a frequency corresponding to a wavelength of an optical signal to be selected to the above AOFT 50. According to this technique, a frequency and an amplitude of an outputted RF signal are controlled according to control signals (frequency control signal, amplitude control signal) from the control unit 52.
The control unit 52 controls a frequency and an amplitude of the RF signal oscillated by the RF oscillator 51. When altering an optical signal at a wavelength to be selected, the control unit 52 outputs an amplitude control signal to suppress an amplitude of the RF signal of the RF oscillator 51 to zero or sufficiently small, after that, outputs a frequency control signal to switch to a predetermined frequency, and stops an output of the above amplitude control signal, thereby recovering the above amplitude.
In order to prevent an optical signal at another wavelength from being selected while a frequency of the RF signal is altered, it is possible that a switch (SW) 53 is provided on the outputting side of the above RF oscillator 51 as indicated by a broken line in FIG. 15, and the control unit 52 controls the switch 53 to turn it OFF when the RF signal outputted from the RF oscillator 51 is altered to another RF signal.
With the above structure, the optical wavelength selective control apparatus 100 makes an amplitude of the RF signal to be supplied to the AOTF 50 sufficiently small when altering a wavelength to be selected by the AOFT 50, then changes an oscillation frequency of the RF oscillator 51 to a frequency corresponding to the wavelength to be selected. Namely, while altering a frequency of the RF signal to the AOTF 50, the optical wavelength selective control apparatus 100 controls an amplitude of the RF signal in order to prevent the AOTF 50 from selecting a signal other than a signal to be selected.
Even if an amplitude of the RF signal to be supplied to the AOTF 50 is suppressed to zero or sufficiently small, or an output of the RF signal is stopped using the switch 53, as above, there is a possibility that another signal is allowed to be transmitted when an extinction ratio of the AOTF 50 is insufficient. The WDM transmission requires approximately 20 to 25 dB as an extinction ratio of the AOTF 50, for example. However, the actual circumstances is that 20 dB is not realized at present. For this reason, there is a good chance of occurrence of the above phenomenon.
If the AOTF 50 is used in an optical network, an insufficient extinction ratio might affect transmission characteristic as coherent crosstalk, leading to degradation of the transmission characteristic. To suppress the affect, at least 45 dB or so is required as an extinction ratio.
In the light of the above problem, an object of the present invention is to provide an optical wavelength selective control apparatus which can select (extract) optical signals at arbitrary wavelengths highly accurately by cascading wavelength selective optical filters so as to largely improve an extinction ratio in a wavelength selecting process, and avoid an effect on optical signals at wavelengths not selected when altering a plurality of wavelengths of optical signals to be extracted to other optical signals.
The present invention therefore provides an optical wavelength selective control apparatus comprising a plurality of wavelength selective optical filters each for extracting an optical signal at an arbitrary wavelength from wavelength-division-multiplexed optical signals obtained by wavelength-division-multiplexing optical signals at a plurality of wavelengths according to a frequency signal for selecting a wavelength, the wavelength selecting optical filters being connected to one another to form a cascade, and a frequency oscillator for outputting a signal at a frequency corresponding to the wavelength of the optical signal to be extracted in each of the wavelength selective optical filters as the frequency signal for selecting a wavelength.
The optical wavelength selective control apparatus according to this invention can perform a wavelength selecting process on optical signals at the same wavelength plural times so that a quality of the selected optical signal is largely improved.
The optical wavelength selective control apparatus according to this invention may further have a control unit for controlling a frequency of the frequency signal outputted from the frequency oscillator, and a first stopping unit interposed between the frequency oscillator and a wavelength selective optical filters to be able to stop an output of the frequency signal from the frequency oscillator, wherein the control unit controls the first stopping unit to stop an output of the frequency signal while the control unit alters a frequency of the frequency signal outputted from the frequency oscillator.
According to this invention, since an output of the frequency signal to the wavelength selective optical filters is stopped while a frequency of the frequency signal outputted from the frequency oscillator is altered, it is possible to prevent other wavelengths than wavelengths to be extracted from being selected even when an oscillation frequency of the frequency oscillator is continuously altered, thus further improve accuracy of the wavelength selecting process.
In the optical wavelength selective control apparatus of this invention, the plurality of wavelength selective optical filters may have the same temperature characteristics respectively and be configured as one common module, and the control unit may control the frequency oscillator according to a change in temperature of the common module to be able to adjust a frequency of the frequency signal.
Alternatively, the plurality of wavelength selective optical filters may have different temperature characteristic and be configured as one common module, and a plural number of the frequency oscillators and the first stopping units may be provided correspondingly to the plurality of wavelength selective optical filters, wherein the control unit controls each of the first stopping units and controls each of the frequency oscillators according to a change in temperature of the common module and the temperature characteristics of each of the wavelength selective optical filters to adjust a frequency of each frequency signal.
Still alternatively, the plurality of wavelength selective optical filters may be configured as individual modules, and a plural number of the frequency oscillators and the first stopping units may be provided correspondingly to the plurality of wavelength selective optical filters, wherein the control unit controls each of the first stopping units and controls each of the frequency oscillators according to a change in temperature of each of the individual modules to adjust a frequency of each frequency signal.
It is possible to output a high-accurate frequency signal from the frequency oscillator at all times since a frequency of the frequency signal from each of the frequency oscillator is adjusted according. to a change in temperature of a module (wavelength selecting unit) having the wavelength selecting optical filter. As a result, accuracy of the wavelength selecting process in each of the wavelength selective optical filters may be further improved.
Still alternatively, the plurality of wavelength selective optical filters may have the same temperature characteristics and be configured as a common module, and the control unit may control a temperature of the common module such that a temperature of the common module is a predetermined temperature.
Further, the plurality of wavelength selective optical filters may have different temperature characteristics and be configured as one common module, and a plural number of the frequency oscillators and the first stopping units may be provided correspondingly to the plurality of wavelength selective optical filters, wherein the control unit controls each of the first stopping units, controls a temperature of the common module so that a temperature of the common module is a predetermined temperature, and controls each of the frequency oscillators according to the temperature characteristics of each of the wavelength selective optical filters to adjust a frequency of each frequency signal.
Alternatively, the plurality of wavelength selective optical filters may have different temperature characteristics and be configured as individual modules, and the control unit may control a temperature of each of the individual modules so that a temperature of each of the individual modules is a predetermined temperature.
Since the control unit controls a temperature of each of the wavelength selective optical filters (wavelength selecting unit) so that a temperature of the module (wavelength selecting unit) having the wavelength selective optical filters is a predetermined temperature, the wavelength selective filter can thereby perform the wavelength selecting process under a stable temperature condition at all times, which further improves accuracy of the wavelength selecting process in the wavelength selective optical filter (wavelength selecting unit).
The first stopping unit may be configured as a switch being able to stop an output of the frequency signal from the frequency oscillator, or an amplifier being able to stop an output of the frequency signal by adjusting an amplification factor for the frequency signal from the frequency oscillator.
It is therefore possible to realize a function of stopping an output of the frequency signal to the wavelength selective optical filters while the above frequency is altered, with an extremely simplified structure.
Alternatively, the optical wavelength selective control apparatus described in claim 1 according to this invention may have a control unit for controlling a frequency of the frequency signal outputted from the frequency oscillator, and a second stopping units each disposed on the outputting side of a corresponding wavelength selective optical filter to be able to stop an output of an optical signal extracted by the wavelength selective optical filter, wherein the control unit controls the second stopping units to stop outputs of the wavelength selective optical filters while the control unit alters a frequency of the frequency signal outputted from the frequency oscillator.
It is thereby possible to stop outputs of optical signals extracted in the wavelength selective optical filters while a frequency of the frequency signal outputted from the frequency oscillator is altered so that transmission of optical signals at wavelengths not selected is certainly prevented, thus an optical signal at a wavelength to be extracted is extracted highly accurately.
Each of the wavelength selective optical filters may be configured with an acouto-optical tunable filter. In which case, it is possible to realize the above-mentioned wavelength extracting function quite easily.
The present invention further provides an optical wavelength selective control apparatus comprising a wavelength selecting unit for extracting optical signals at a maximum of m (m is a natural number satisfying 1 less than m less than N) wavelengths from wavelength-division-multiplexed optical signals obtained by wavelength-division-multiplexing optical signals at N (N is a natural number not less than 2) wavelengths according to frequency signals for selecting wavelengths, m frequency oscillators for outputting signals at frequencies corresponding to wavelengths of the optical signals to be extracted in the wavelength selecting unit as the frequency signals for selecting wavelengths, a control unit for controlling each of frequencies of the frequency signals outputted from the frequency oscillators, and m stopping units each being able to stop an output of the frequency signal from a corresponding frequency oscillator, wherein the control unit controls each of the stopping units to stop an output of the frequency signal while the control unit alters frequencies of the frequency signals outputted from the frequency oscillators.
The above apparatus stops outputs of the frequency signals to the wavelength selecting unit by the stopping units when altering selected wavelengths, then alters frequencies from the frequency oscillators that are targets in the frequency alteration to desired values, so that another optical signal not selected does not leak as a selected optical signal when selected wavelengths are altered. Therefore, it is possible to extract optical signals at wavelengths to be extracted highly accurately, thus improve a performance of the wavelength-division-multiplexing optical transmission system. In this case, a required number of the oscillators (m) is less than the number of times of multiplexing (N) of the wavelength-division-multiplexed optical signal, which simplifies a structure of the apparatus.
The wavelength selecting unit may be configured as a wavelength selective optical filter for extracting optical signals at arbitrary m wavelengths from the wavelength multiplexed optical signals obtained by multiplexing N wavelengths according to frequency signals from the m frequency oscillators. In which case, it is possible to realize an m-wavelength selecting function with an extremely simple structure.
The wavelength selecting unit may have a plurality of wavelength selective optical filters each for extracting optical signals at arbitrary m wavelengths from the wavelength-division-multiplexed optical signals according to the frequency optical signals from the m frequency oscillators, wherein the wavelength selective optical filters are connected to one another to form a cascade.
In this case, it is possible to perform the wavelength selecting process on optical signals at the same wavelength plural times so that a quality of the selected optical signal is largely improved.
In this case, if the wavelength selective optical filter is configured with an acousto-optic tunable filter, it is possible to realize the above wavelength selecting function quite readily.
The control unit may control the frequency oscillators according to a change in temperature of the wavelength selecting unit so as to adjust frequencies of frequency signals.
Accordingly, it is possible to output high-accurate frequency signals at all times from the frequency oscillators, which further improves accuracy of the wavelength selecting process in the wavelength selecting unit.
Further, the control unit may control a temperature of the wavelength selecting unit such that a temperature of the wavelength selecting unit is a predetermined temperature.
The wavelength selecting unit can thereby perform the wavelength selecting process under a stable temperature condition at all times so that it is unnecessary to finely adjust an oscillation frequency of the frequency oscillator, as above.
Each of the stopping units may be configured as a switch controlled by the control unit to be turned OFF to be able to stop an output of the frequency signal from a corresponding frequency oscillator, or as an amplifier being able to stop an output of the frequency signal by adjusting an amplification factor of the frequency signal from a corresponding frequency oscillator by the control unit.
It is therefore possible to realize a function of stopping an output of the frequency signal to the wavelength selecting unit while the selected wavelength is altered, with an extremely simple structure.
The present invention still further provides an optical wavelength selective control apparatus comprising a wavelength selecting unit for extracting optical signals at arbitrary m (m is a natural number satisfying 1 less than mxe2x89xa6N) wavelengths from wavelength-division-multiplexed optical signals obtained by wavelength-division-multiplexing optical signals at N (N is a natural number not less than 2) wavelengths according to frequency signals for selecting wavelengths, N frequency oscillators for outputting signals at frequencies corresponding to wavelengths of the optical signals to be extracted in the wavelength selecting unit as the frequency signals for selecting wavelengths, N stopping units each for stopping an output of the frequency signal from a corresponding frequency oscillator, and a control unit for controlling relevant stopping units to transmit outputs of the frequency signals corresponding to wavelengths that are targets in alteration, while controlling relevant stopping units to stop outputs of the frequency signals other than the frequency signals corresponding to the wavelengths that are targets in alteration to when altering wavelengths of optical signals to be selected in the wavelength selecting unit.
Therefore, it is possible to alter selected wavelengths in the wavelength selecting unit only by controlling each of the stopping units be turned ON/OFF, so that accuracy of optical signals at wavelengths to be extracted may be improved and the wavelength selecting process may be performed quickly.
The wavelength selecting unit may be configured as a wavelength selective optical filter for extracting optical signals at arbitrary N wavelengths from the wavelength-division-multiplexed optical signals according to the frequency signals from the N frequency oscillators. In which case, it is possible to accomplish the above wavelength selecting function with an extremely simple structure.
Alternatively, the wavelength selecting unit may have a plurality of wavelength selective optical filters each for extracting optical signals at arbitrary m wavelengths from the wavelength-division-multiplexed optical signals according to the frequency signals from m frequency oscillators among the N frequency oscillators, wherein the wavelength selective optical filters are connected to one another to form a cascade.
In this case, it is possible to perform the wavelength selecting process on optical signals at the same wavelength plural times so that a quality of selected optical signals is largely improved.
The wavelength selective optical filter may be configured with an acousto-optic tunable filter. In which case, the above-mentioned wavelength selecting function may be accomplished quite readily.
The control unit may control the frequency oscillators according to a change in temperature of the wavelength selecting unit to adjust frequencies of the frequency signals.
It is thereby possible to output high-accurate frequency signals from the frequency oscillators at all times, thus further improve accuracy of the wavelength selecting process in the wavelength selecting unit.
The control unit may control a temperature of the wavelength selecting unit such that a temperature of the wavelength selecting unit is a predetermined temperature.
It is thereby possible to perform the wavelength selecting process in the wavelength selecting unit under a stable temperature condition at all times so that accuracy of the wavelength selecting process in the wavelength selective optical filter (wavelength selecting unit) is further improved.
Each of the stopping units may be configured as a switch controlled by the control unit to be turned OFF to be able to stop an output of the frequency signal from a corresponding frequency oscillator, or an amplifier being able to stop an output of the frequency signal by adjusting an amplification factor for the frequency signal from a corresponding frequency oscillator by the control unit. In which case, it is possible to stop outputs of the frequency signals for optical signals not selected to the wavelength selecting unit while selected wavelengths are altered, with an extremely simple structure.